The conventional nuclear reactor power plant comprises a pressure vessel structure which houses, among others, the nuclear core, subassemblies and a fluid coolant for direct or indirect steam generation for conventional electrical power production. Moreover, in certain new systems, it has been suggested to dispose modular heat exchanger units about the inner wall of the pressure vessel in heat exchange relationship with the nuclear core coolant. Secondary fluid flowing through the modular units is converted to steam therein and is directed to electrical power generation means outside of the reactor pressure vessel.
In each of the above described systems, however, the pressure vessel must have inlet penetrations through which either fresh primary core coolant or secondary fluid or secondary coolant enters the pressure vessel to be heated to an elevated temperature. Because this incoming coolant is relatively cold, in contrast to the higher temperature of the thick-walled reactor pressure vessel, not only is there a risk of initiating boiling in the coolant that flows through the inlet, but there also is a risk of establishing unacceptably high stresses within the reactor pressure vessel wall because of temperature related differences in the expansion of the metal in the vessel that is adjacent to and spaced from the cold feedwater inlet.
Furthermore, because of the safety requirements and the extreme costs inherent in a nuclear power plant, an inlet nozzle penetration of the reactor pressure vessel requires special consideration. For example, expensive cladding such as Inconel is generally used to line the inlet penetration bores through the reactor pressure vessel and thereby prevent corrosion of the vessel wall. Furthermore, the restraint stresses developed between the vessel wall and the attached inlet nozzle penetration fittings resultings from thermal and pressure differences between parts usually is a compromise, especially under transients caused by malfunction of feedwater heaters and thermal cycles in operation. In addition, for safety purposes, it is necessary to arrange the inlet nozzle penetration attachment welds such that they meet the requirements of ASME Code Section III. Furthermore, from a cost view point, it is desirable to design nuclear reactor components in a manner which allows practical non-destructive testing thereof and non-destructive inspection, such as x-ray, gamma ray, ultrasonic and penetrant of all the welds.
Accordingly, there is a need to provide an inlet nozzle penetration means which alleviates the thermal stress briefly discussed above, arranges the nozzle penetration welds to conform with ASME codes and arranges the inlet nozzle penetration and the welds associated therewith in such a manner as to allow non-destructive testing and inspection thereof.